Regional differences of metabolism in human masseter muscle by two-dimensional P-31-chemical shift imaging

Many reports have demonstrated significant region-dependent differences in the fiber-type composition of the human masseter muscle. Therefore, it is c onsidered that there is intramuscular heterogeneity of metabolic activity i n the muscle. The present study was carried out, with two-dimensional Chemi cal Shift Imaging, to detect differences between the deep and superficial p arts of the human masseter muscle at rest. Masseter muscle from 11 voluntee rs, from 20 to 27 years old, was examined, and characteristic spectra of th e inorganic phosphate (Pi), phosphocreatine (PCr), and alpha-, beta-, and g amma-adenosine triphosphate (ATP) from each part of the muscles were obtain ed. In this study, the deep and superficial parts of the masseter muscle we re distinguished by the existence of aponeurosis. The Pi/PCr, PCr/beta-ATP, and Pi/beta-ATP ratios as well as the pH in the deep and superficial parts were calculated from the peak spectra. Compared with the deep part, the Pi /PCr of the superficial part was lower (p < 0.05) and the PCr/beta-ATP was higher (p < 0.01). The Pi/beta-ATP and pH showed no significant differences between the two parts. The results indicate that the superficial part of t he masseter muscle contains more PCr than the deep part, and this may be re lated to functional differences between these two parts. In future examinat ions of the metabolic activity of the human masseter muscle, the deep and s uperficial parts must be measured separately.